1. Field of the Invention
This invention relates to devices used to provide a dimming function for ordinary single filament incandescent light bulbs. It is specifically designed to fit into a screw-in light bulb socket such as an Edison socket, although, with appropriate dimensions, the device could be used for other screw-in sockets such as Mogul sockets and miniature sockets.
2. Description of the Prior Art
Screw-in sockets have been in common use, probably from the advent of Edison's first commercialization of his incandescent light bulb. At that time, i.e., in the early days of commercialization of electric power, Edison sockets were used not only for light bulbs, but also for "plugging in" other electrical appliances which were attached via line cords to Edison plugs. While blade or pin connectors are presently used for line cords, most incandescent light bulbs in the home are installed by screwing the bulb into an Edison socket or a similar screw-in socket.
Several devices are presently commercially available which control current for dimming light bulbs. One type of device, represented by U.S. Pat. Nos. 3,450,893; 3,818,263 and 3,823,339, uses a diode rectifier sandwiched between a pair of metal disc contacts. The contacts and rectifier are surrounded by a ring of insulating material which separates the contacts from each other and the socket's outer conductor. The assembly is placed in a lamp socket between the center conductor of a light bulb and the bottom conductor of the socket and operates by cutting off the top halves of AC line current. This simultaneously dims the bulb, reduces temperature and extends the bulb's lifetime.
Other commonly available devices have a male Edison connector on one end, a female Edison connector on the other end, and have dimmer and switching circuitry between the connectors. These devices are insertable into an Edison socket, with the bulb being separately inserted into the other end of the device. Thus, in the prior art, it has been necessary to have the bulb physically separate from the socket in order to provide a switchable dimmer circuit for a single-filament bulb.
In addition to the common four-position lamp switch for two-filament ("3-way") bulbs, special sockets are available which are capable of providing a dimmer function for a single filament bulbs. These, of course, have to be either provided with the lamp or installed by someone who has the knowledge necessary to attach the electrical wires to a lamp. While this is a simple mechanical operation, there are many people who cannot safely accomplish such a project. One available device, produced by Leviton, is insertable into a lamp's line cord by clamping onto the cord and is capable of switching the lamp between full power, off and rectifier-attenuated power. While the device is fairly simple to install, the installation is at least as permanent as the electric wire itself and is, of course, separate from the bulb.
The devices which use rectifiers take advantage of the incandescent lamp's positive temperature cooefficient of resistance. This enables the lamp to glow at partial power even though it has been deprived of a substantial portion of the AC current wave form. Applying the formula IE=W, the current at full power can be determined by: EQU I=W/E
where
I=current
E=voltage
W=power.
As an example, for a 60 watt bulb operating at 120 volts. the current would be: EQU I=60/120=0.5 amp.
The resistance of such a bulb can be determined by Ohm's law, IR=E, or R=E/I.
In the specific example, the bulb's resistance at rated voltage would be: EQU R=120 v/0.5 amps.=240.OMEGA.
When current is reduced, the PTC nature of the bulb's filament reduces resistance and thereby allows an amount of current to pass through the bulb proportionately greater than the percentage of voltage available. Thus, the amount of power consumed by the bulb when available current is reduced by AC rectification would be greater than that anticipated by the above formula because the bulb's resistance is less, causing the current to be greater than if the resistance were the same.